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There is very little information on the history and development of valve emulation designs, for several reasons:
Successful modelling will reproduce the sound of a valve output stage at various overdrive levels into different guitar speaker systems. Most modellers include digital effects as well.
Digital modelling is the most popular approach to emulating guitar amplification systems at present. There have been attempts to recreate this sound with analogue designs and hybrid approaches as well. Of course, overdrive pedals are analogue attempts to create valve overdrive tones.
The biggest challenge (and weaknesses of cheaper units) are to accurately capture the transition of a valve power amplifier from clean into light overdrive and beyond into heavy overdrive. This allows players to dig in for more aggressive sounds, or lay back for cleaner tones. A good valve amplifier into a good guitar speaker cabinet does this with a remarkable amount of "musicality". Notes within chords remain clear, and the distortion (buzz) characteristic is musically related without being over-hyped.
There is also some mild compression, as notes start clipping, and decay into the clean tone with very little volume reduction. These dynamics give an amplifier a 'feel' where the guitar and amplifier inter-react in what many players describe as an "organic" way.
Whether you use a Fender, Marshall, Boogie, Vox or whatever, they each produce their own distinctive valve tone. The variations are due mainly to the output stage design (how "hot" the valves are biased), whether negative feedback is used, and different speaker types and cabinet designs.
I have attempted to put a little history together here, please let me know if you have any additional information I can add by emailing me. The emulations fall generally into 3 camps: valve pre-amp overdrive, solid state and digital.
Mesa Boogie have really set the pace here, with designs born from hot-rodded Fenders. In addition to their multi-stage pre-amps in their regular amp range, they also produced a sophisticated rack pre-amp (the Triaxis). These pre-amps use multiple valve stages interspersed with tone filtering and adjustable gain to produce a wide variety of useful overdrive tones. These designs kept the guitar signal all analogue, but included simple digital switching and control systems.
Of course, it didn't take long for all amps to include a master volume control and extra gain stages to allow preamp overdrive. Most early attempts were harsh, with little tone compensation for the high frequency fuzz generated by this overdrive. Regardless of manufacturer, in my opinion, these circuits at their best produce some great tones, but do not deliver all of the vital components of output stage overdrive.
Several high-end manufacturers, such as Soldano, specialise with their own signature valve pre-amp overdrive tones.
From the first solid state amplifier, manufacturers have believed it is possible to emulate valve tone. As mentioned elsewhere in my pages, early attempts were dismal, and have given this genre a bad image. Nevertheless, there have been several significant developments.
Early attempts were made with various stomp-box effects. These are overdriven solid state pre-amps.
All of these pedals are still available today. Probably the most valve-like are the overdrive pedals, although these really don't have the note clarity or dynamics of the real thing.
There has been much hype about the Ibanez Tube Screamer being the 'valve pedal'. Players like Stevie Ray Vaughan actually used this pedal as a signal booster (with Drive set near minimum, Level set near maximum) to overdrive their valve amplifiers harder. By using a cleaner tone on their amplifier, you can use pedals like this for a solo boost footswitch that also overdrives the amp.
There have been other more sophisticated emulations. The Tech 21 Sans Amp range uses solid state circuitry to produce some great valve-like tones, and some models claim to include speaker emulations to further impart distinctive sounds. Most reviews I have read praise the flexibility and overdrive characters, but say the valve dynamic feel is missing. This is not a criticism; the Tech 21 and other models in this category are capable of providing more controlled and different valve tones than you could ever get out of a valve amp.
Several other manufacturers have produced similar distinctive tones. The Marshall JMP-1 produces many Marshall-like tones (as you would expect). This rack unit actually includes a valve, but like their controversial JCM-900 range, includes solid state clipping for the main part of the overdrive tone.
There is an irony: Valve power amps have no special or complex design (other than being over-engineered for reliability); they were built the way all audio amps were built at the time, and just happen to sound sensational for guitar. On the other hand, a 'text book' solid state power amp sounds simply awful when used by guitarists in the same way. The design needs to be moderated and controlled in so many ways that it is not surprising progress has been so slow.
The Marshall Valvestate and Peavey Transtube designs go some way towards combining both valve feel and tone. To my ears, the Peavey produces a more modern 'gritty' overdrive tone, while the Marshall has a creamier overdrive. These amps are significantly better than the earlier solid state amps we've loved to hate, but are still not quite the real thing. Considering their price and other advantages, they are nevertheless an excellent buy.
There have been several non-mainstream efforts to produce both the tone and feel of valve designs:
The Maven Peal Sag Circuit is not an emulation circuit, it is actually a power supply designed to sag like vintage valve amps using valve rectifiers. The circuit also allows guitarists to control the level of sag, from 0% to 100%. I have not heard this circuit, but I would expect it offers the dynamic compression of these original amplifiers. "Entry level" valve amplifiers today use solid state rectifiers, with a harder, more aggressive sound than vintage amps. The Sag Circuit circuit works with both valve and transistor amplifiers, and for transistor amps, musicians can adjust the maximum power level the power amp can produce, allowing it to be overdriven at lower volume levels. If you're interested in trying it out, you can email them, or write to 1947 Nebraska Valley Road, Stowe, Vermont 05672.
Another design comes from Eric Pritchard. He has worked on getting transistors to sound right for many years and has several patents. He is looking to go into production, so if you can help, email Eric.
And last but not least, there's GM Arts (that's me!). I've spent over 40 part-time years researching, designing and building tube emulation overdrive pedals, aiming for valve tone and feel. I would say I've made good progress, and certainly have some very natural sounds with designs never used before. But I don't expect to put these into production because good digital modellers are now closer to the real thing than my designs.
These circuits convert your signal to the digital domain (zeros and ones). This signal is then manipulated by computer algorithms (programs) before being converted back to analogue sound. Like all approaches, there are advantages and disadvantages. The computer can do many things to the digital sound stream, such as valve pre-amp tone control, power amp overdrive, speaker and microphone emulation, and digital effects (like chorus, delay and reverb) all at the same time!
The Roland VG-8 is an emulation of the entire guitar as well, but Roland later applied the same technology to produce the rack GP-100 processor (and even more recent Boss GX-700 and GT floorboard variations). I used the GP-100 for many years, and found it flexible and capable of many great tones.
Line 6 are a relatively new manufacturer producing amps with different valve emulations. Their new Flextone amps have set a standard in amp flexibility and simplicity; it's like a regular amp with tone, gain and volume controls, and has just 2 extra dials to select the amplifier type you want, and the effects you want to add. Their POD recording preamps can be used live as well. In my view, the original POD evolved into a valuable live overdrive preamp, while later releases (the XT and X3) improved on authenticity, particularly with clean and lightly overdriven tones. Several users have complained of an unmusical high-end grittiness not present in the real thing.
Yamaha released their DG-1000 pre-amp (and also an amplifier using the same pre-amp). This unit features motorised dials which move to their stored settings when you change patches! It doesn't include any other digital effects, but has received favourable reviews for its amp emulations.
Just about every major amp/effects manufacturer has a digital modelling range now.
An interesting point worth noting (raised by a reader; thanks David), is that because the sound of overdriven components is generated in a tone-emulating pre-amp, it is hugely detrimental to feed this signal into an overdriven solid state power amp. All the good work is undone! There are several ways around this:
The same alternatives apply to speakers. A traditional good guitar speaker system rolls off the bass gradually below about 100Hz, and highs are rolled off abruptly in the 4KHz to 8KHz octave, with some significant peaks and troughs above about 1KHz. This would colour an emulated sound, specially one doing its own speaker emulations, which might be trying to add peaks where your own speakers have troughs!
Celestion speakers have a great reputation for providing good signature guitar sounds, while EVs are better for providing a flatter and less coloured sound. If your modeller is able to model guitar cabs well enough, you can use a powered PA monitor which have a much flatter response (called FRFR for full range, flat response).
Opinion is divided on this issue; some players prefer to have a coloured basic sound with their power amp and speakers, and use emulations to provide subtle variation in amp 'vibe' and overdrive levels. Other players prefer to keep the power stage pretty clean, and let the emulations do the work.
Also, opinions vary on the success of digital emulation. In my opinion, the semi-pro products (such as those from Line 6, Digitech and Vox) sit somewhere between the best solid state designs and the real thing. Fractal Audio released their Axe-FX product around 2005 and this has evolved into easily the best hardware modeller at the time writing. This product, combined with 3rd party cabinet emulations (such as those from Red Wire) easily sits alongside a good valve amplifier for realism, and beats it for flexibility.
Digital modelling lends itself to software plug-ins into recording packages. These also vary in realism, with Peavey Revalver seeming to get the best and most consistent reviews at the time of writing.